The present invention relates to a circuit arrangement and a method for detecting a capacitance of a capacitive component and/or a change in the capacitance of a capacitive component.
A circuit arrangement of the generic type is known from DE 10 2006 029 120 A1. In the circuit arrangement disclosed in said document, a control unit for controlling a monostable flipflop is provided, which is connected to the capacitive component, for example a sensor electrode or an electrode arrangement comprising a response electrode and a reference electrode. The control unit is connected to a first input of the monostable flipflop in order to transmit control signals to the monostable flipflop, and the capacitive component is connected to a second input of the monostable flipflop. An output signal is generated by a control signal at an output of the monostable flipflop, the duration of said output signal being dependent on the capacitance of the capacitive component. Such an output signal is converted into a signal voltage via a conversion device, the course of said signal voltage being proportional to the duration of the output signal. The signal voltage obtained is thus likewise proportional to the capacitance of the capacitive component or a change in capacitance, with the result that it is possible to draw a conclusion in respect of the capacitance or the change in capacitance on the basis of the signal voltage. The signal voltage is correspondingly evaluated by an evaluation unit in order to generate from this a detection value which indicates the present capacitance and/or a change in the capacitance of the capacitive component.
Such a circuit arrangement is usable, for example, for an adjusting device for a closing element of a vehicle which is adjustable, actuated by external force, for example for a tailgate or side door which is adjustable by an electric motor or a boot lid of a motor vehicle which is adjustable by an electric motor. The capacitive component is in this case formed, for example, by a sensor electrode in order to be able to detect an operation event contactlessly hereby. Thus, the measurable capacitance changes owing to a body part of a user in the region of the sensor electrode, with the result that a user can initiate an operation event in a targeted manner by the respective body part, which operation event is detected via the sensor electrode and should result in automatic adjustment of the respective closing element. An exemplary application in this case is the application of a sensor electrode in the region of a rear bumper of a motor vehicle in order to identify, via the sensor electrode, a movement of a foot of a user in the region of the bumper in the manner of a kick as an operation event and to open the tailgate or the boot lid of the motor vehicle in response to this. The adjustment of the closing element in the form of the tailgate or the boot lid can therefore be controlled contactlessly, i.e. without the respective closing element being touched by the user.
DE 10 2006 029 120 A1 proposes a conversion device having a constant current source and an integrator in the form of a capacitor. In this case, the switching-on and switching-off of the constant current source or the connection thereof to the integrator is controlled via the output signal of the monostable flipflop. Depending on the duration of the output signal, which is in turn dependent on the capacitance of the capacitive component, a signal voltage is thus integrated at the integrator by the constant current source, i.e. the integrator is charged in the form of a capacitor. The signal voltage which is measurable via the integrator is evaluated by an electronic evaluation unit in order to draw a conclusion in respect of the presence of an operation event, for example when a threshold value is exceeded, and to generate a tripping signal. The adjustment of a closing element of a motor vehicle would then be controllable via such a tripping signal, for example.
The provision of a constant current source and the targeted switching thereof on the basis of the output signal provided by the monostable flipflop, for example via a switch to be provided in the circuit arrangement, can under certain circumstances be only comparatively complex to implement in terms of circuitry, however.
DE 102 58 417 B3 again describes a circuit arrangement for detecting and/or determining the properties of a medium with the aid of a capacitive component in the form of a pair of electrodes, in which a conclusion is drawn in respect of a composition of a medium into which the two electrodes are immersed on the basis of the capacitance measurable via the two electrodes. For this purpose, the two electrodes are connected to a monostable flipflop, referred to as monostable multivibrator in DE 102 58 417 B3, which generates an output signal with a specific duration depending on the capacitance at the electrodes. This output signal is passed via a low-pass filter connected downstream of the monostable flipflop in order thereafter to provide a signal voltage which is evaluated by an evaluation unit. In this case, for example, the content of water in the respective medium can be determined on the basis of the value of the signal voltage, and it is possible to distinguish between different media, such as, for example, diesel fuel, biodiesel fuel and water on the basis of their different dielectric constants.
In the circuit arrangement from DE 102 58 417, however, it is disadvantageous when using an RC low-pass filter that the signal voltage which can be tapped off at a capacitor of the low-pass filter only has a temporarily linear course over time, to be precise only for as long as an input voltage present at the low-pass filter is considerably lower than the output voltage which can be measured at the output of the low-pass filter. The output voltage has an e function over time in the case of an RC low-pass filter and therefore asymptotically approaches a maximum value. Precisely in order to detect whether an operation event for adjusting a closing element of a vehicle is present, however, a linear relationship between the capacitance of the capacitive component and the signal voltage also over relatively long time periods is preferred in order to be able to therefore safely identify the respective, possibly complex operation event.